Electrical Power Calculator
The appliance's appetite: P = V·I from your rail and draw, the power family re-derived three ways as a check, and a day of running priced in kilowatt-hours.
Electrical Power Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Resistive, unity-power-factor reading
- Flat draw over time — no duty cycles
In short: A kettle's worth on a 120 V rail drawing 10 A: P = V·I = 120 × 10 = 1,200.000000 W. The family card re-derives the same wattage three ways from your numbers — R = V/I = 12.000000 Ω, so P = I²R = 1,200 W and P = V²/R = 1,200 W: three roads, one watt. A day of running costs 28.800000 kWh (103,680,000 J) on the meter — watts are a RATE, and the meter runs on the product of rate and time. Move the same draw to a 230 V rail and the appetite doubles to 2,300.000000 W.
Formula
P = V·I = I²R = V²/R · kWh = W·h / 1,000 · 1 kWh = 3,600,000 J (exact)
Power is the RATE of energy: watts are joules per second, and the volt-ampere product is one of three equal readings of it once resistance exists. The meter does not sell watts, though — it sells watt-hours, which is why the day card multiplies before it divides. The three-roads card is a built-in lie detector: whatever two facts you hold, all three formulas must land on the same watt.
Worked Example
- Enter the rail voltage and the draw in amperes.
- Read the wattage, then watch the family re-derive it three ways.
- Read a day's energy in kWh — the unit your bill actually speaks.
- Move the load across rails with the chips to feel what voltage does to appetite.
Defaults: 1,200.000000 W; three roads agree; a day = 28.800000 kWh. On 230 V: 2,300.000000 W.
Strengths & Limits Of This Model
Where this engine is strong
- Power family derived three ways from your inputs
- Day card in kWh, the unit bills speak
Where it stops
- No power factor or inrush
- No tariff or cost modelling
Practical Use Cases
Household
what each appliance costs to run
Solar sizing
loads against panel budgets
Teaching
the power family, checked three ways
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Electrical Power Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why three formulas for the same power?
They are one formula wearing substitutions: P = V·I, with V = IR substituted to give I²R, or I = V/R to give V²/R. The page derives all three from YOUR numbers so the agreement is something you watched, not something you were told.
Why does the page print zero for I = 0?
Because nothing drawing is a real, measurable state: P = 120 × 0 = 0 W, honestly. (The voltage page refuses a zero current because it DIVIDES by it; this page only multiplies — the arithmetic decides which zeros are answers and which are refusals.)
What is the difference between a watt and a kilowatt-hour?
A rate and an amount. Watts measure joules per second; kilowatt-hours measure the joules that flowed while the rate held for a while: 1 kWh = 3,600,000 J, exactly. The day card makes the trade visible — 1,200 W is also 103,680,000 J by midnight.
Why does the same draw double on a 230 V rail?
Because P = V·I with the current held: double the push and the appetite follows. Real heaters self-limit (their resistance fixes I = V/R, halving the current on the taller rail), which is why travel adapters and dual-rating plates exist.
Where does the billing number actually come from?
Energy: the meter integrates watts over time. The day card assumes the draw holds for 24 hours; a thermostat-cycled appliance runs a duty fraction of that, which is why rated watts and billed kilowatt-hours disagree in every honest energy audit.
Does this work for AC?
For resistive loads, yes — the RMS convention makes volts times amps the watt. Motors and supplies add power factor, where volt-amperes and watts part ways; that correction lives beyond this page's straight-line honesty.
Why is negative current refused?
A sign is a direction, and the appetite does not care which way the electrons march — only how many per second. Enter magnitudes; the page keeps the watts positive and the physics honest.
How does this pair with the current page?
As one circuit read from two ends: that page divides power by voltage to find the draw; this page multiplies voltage by the draw to find the power. The kettle at 1,200 W on 120 V is the same fact wearing both directions.